Wireless LANs and neighborhood capture

ABSTRACT

Overlapped wireless LAN cells in a medium have an equal chance at establishing a session on the medium. A first member station in the first cell transmits a timing packet containing a timestamp value, which is received at a second member station in the second cell. This synchronizes member stations in the first and second cells to interrupt transmissions at a global channel release instant corresponding to the timestamp value. The member stations in the first and second cells then have the opportunity to contend for access to the medium following the global channel release instant, using a slotted CSMA/CA access method. Each of the member stations in the first and second cells has a superframe clock that is synchronized based on the timestamp value, thereby establishing a periodic global channel release instant during each of a plurality of periodic superframes. The member stations can then periodically interrupt transmissions at the periodic global channel release instant to contend for the medium. The periodic global channel release instant occurs at intervals that are sufficiently close to meet delay and jitter restrictions for time-critical voice and video applications.

[0001] This application claims the benefit of the following co-pendingapplications:

[0002] U.S. Provisional Application Serial No. 60/330,930, filed Nov. 2,2001, entitled “HCF ACCESS MECHANISM: OBSS MITIGATION,”

[0003] U.S. Provisional Application Serial No. 60/331,030, Nov. 7, 2001,entitled “‘NEIGHBORHOOD’ CAPTURE IN CSMA/CA WLANS”,

[0004] U.S. Provisional Application Serial No. 60/331,211, filed Nov.13, 2001, entitled “‘SHIELD’: PROTECTING HIGH PRIORITY CHANNEL ACCESSATTEMPTS,” and

[0005] U.S. Provisional Application Serial No. 60/342,343, Dec. 21,2001, entitled “WIRELESS LANS AND ‘NEIGHBORHOOD CAPTURE’,” all of whichare incorporated herein by reference.

RELATED APPLICATIONS

[0006] This patent application is related to the copending regular U.S.patent application Ser. No. 09/985,257, filed Nov. 2, 2001, by MathildeBenveniste, entitled “TIERED CONTENTION MULTIPLE ACCESS (TCMA): A METHODFOR PRIORITY-BASED SHARED CHANNEL ACCESS,” which is incorporated byreference.

[0007] This patent application is also related to the copending regularU.S. patent application Ser. No. 10/187,132, filed Jun. 28, 2002, byMathilde Benveniste, entitled “HYBRID COORDINATION FUNCTION (HCF) ACCESSTHROUGH TIERED CONTENTION AND OVERLAPPED WIRELESS CELL MITIGATION,”which is incorporated by reference.

[0008] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “‘SHIELD’: PROTECTING HIGH PRIORITY CHANNEL ACCESSATTEMPTS IN OVERLAPPED WIRELESS CELLS,” which is incorporated byreference.

[0009] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “PREEMPTIVE PACKET FOR MAINTAINING CONTIGUITY INCYCLIC PRIORITIZED MULTIPLE ACCESS (CPMA) CONTENTION-FREE SESSIONS,”which is incorporated by reference.

[0010] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “FIXED DETERMINISTIC POST-BACKOFF FOR CYCLICPRIORITIZED MULTIPLE ACCESS (CPMA) CONTENTION-FREE SESSIONS,” which isincorporated by reference.

[0011] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “ACCESS METHOD FOR PERIODIC CONTENTION-FREESESSIONS,” which is incorporated by reference.

[0012] This patent application is also related to the copending regularU.S. patent application Ser. No. ______, filed ______, 2002, by MathildeBenveniste, entitled “STAGGERED STARTUP FOR CYCLIC PRIORITIZED MULTIPLEACCESS (CPMA) CONTENTION-FREE SESSIONS,” which is incorporated byreference.

FIELD OF THE INVENTION

[0013] The invention generally relates to the field of communicationsand specifically to a system and method for reducing the effects ofchannel capture for extended periods of time in multiple-cell wirelesslocal area networks (WLANs), thus, improving quality of service (QoS).

BACKGROUND OF THE INVENTION

[0014] A single-cell wireless LAN using the IEEE 802.11 Wireless LANStandard is a Basic Service Set (BSS) network. When all of the stationsin the BSS are mobile stations and there is no connection to a wirednetwork, it is an independent BSS (IBSS). An IBSS has an optionalbackbone network and consists of at least two wireless stations. Amultiple-cell wireless LAN using the IEEE 802.11 Wireless LAN Standardis an Extended Service Set (ESS) network. An ESS satisfies the needs oflarge coverage networks of arbitrary size and complexity.

[0015] The IEEE 802.11 Wireless LAN Standard is published in three partsas IEEE 802.11-1999, IEEE 802.11a-1999, and IEEE 802.11b-1999, which areavailable from the IEEE, Inc. web sitehttp://grouper.ieee.org/groups/802/11. The IEEE 802.11 Wireless LANStandard defines at least two different physical (PHY) specificationsand one common medium access control (MAC) specification. The IEEE802.11(a) Standard is designed to operate in unlicensed portions of theradio spectrum, usually either in the 2.4 GHz Industrial, Scientific,and Medical (ISM) band or the 5 GHz Unlicensed-National InformationInfrastructure (U-NII) band. It uses orthogonal frequency divisionmultiplexing (OFDM) to deliver up to 54 Mbps data rates. The IEEE802.11(b) Standard is designed for the 2.4 GHz ISM band and uses directsequence spread spectrum (DSSS) to deliver up to 11 Mbps data rates.

[0016] Other wireless LAN standards include: Open Air (which was thefirst wireless LAN standard), HomeRF (designed specifically for the homenetworking market), and HiperLAN/2 (the European counterpart to the“American” 802.11a standard). Bluetooth is a personal area network (PAN)standard. It is aimed at the market of low-power, short-range, wirelessconnections used for remote control, cordless voice telephonecommunications, and close-proximity synchronization communications forwireless PDAs/hand-held PCs and mobile phones.

[0017] The IEEE 802.11 Wireless LAN Standard describes two majorcomponents, the mobile station and the fixed access point (AP). IEEE802.11 networks can also have an independent configuration where themobile stations communicate directly with one another, without supportfrom a fixed access point. The medium access control (MAC) protocolregulates access to the RF physical link. The MAC provides a basicaccess mechanism with clear channel assessment, channel synchronization,and collision avoidance using the Carrier Sense Multiple Access withCollision Avoidance (CSMA/CA) access method. The MAC provides linksetup, data fragmentation, authentication, encryption, and powermanagement.

[0018] Synchronization is the process of the stations in an IEEE 802.11wireless LAN cell getting in step with each other, so that reliablecommunication is possible. The MAC provides the synchronizationmechanism to allow support of physical layers that make use of frequencyhopping or other time-based mechanisms where the parameters of thephysical layer change with time. The process involves sending a beaconpacket to announce the presence of a wireless LAN cell and inquiring tofind a wireless LAN cell. Once a wireless LAN cell is found, a stationjoins the wireless LAN cell. This process is entirely distributed inwireless LAN cells and relies on a common timebase provided by a timersynchronization function (TSF). The TSF maintains a 64-bit timer runningat 1 MHz and updated by information from other stations. When a stationbegins operation, it resets the timer to zero. The timer may be updatedby information received in a beacon packet.

[0019] In an independent BSS (IBSS) wireless LAN cell, there is noaccess point (AP) to act as the central time source for the wireless LANcell. In a wireless LAN cell, the timer synchronization mechanism iscompletely distributed among the mobile stations of the wireless LANcell. Since there is no AP, the mobile station that starts the wirelessLAN cell will begin by resetting its TSF timer to zero and transmittinga beacon packet, choosing a beacon period. This establishes the basicbeaconing process for this wireless LAN cell. After the wireless LANcell has been established, each station in the wireless LAN cell willattempt to send a beacon after the target beacon transmission time(TBTT) arrives. To minimize actual collisions of the transmitted beaconframes on the medium, each station in the wireless LAN cell will choosea random delay value, which it will allow to expire before it attemptsits beacon transmission.

[0020] In order for a mobile station to communicate with other mobilestations in a wireless LAN cell, it must first find the stations. Theprocess of finding another station is by inquiry. The inquiring may beeither passive or active. Passive inquiry involves only listening forIEEE 802.11 traffic. Active inquiry requires the inquiring station totransmit and invoke responses from IEEE 802.11 stations.

[0021] Active inquiry allows an IEEE 802.11 mobile station to find awireless LAN cell while minimizing the time spent inquiring. The stationdoes this by actively transmitting queries that invoke responses fromstations in a wireless LAN cell. In an active inquiry, the mobilestation will move to a channel and transmit a probe request frame. Ifthere is a wireless LAN cell on the channel that matches the service setidentity (SSID) in the probe request frame, the responding station inthat wireless LAN cell will respond by sending a probe response frame tothe inquiring station. This probe response includes the informationnecessary for the inquiring station to extract a description of thewireless LAN cell. The inquiring station will also process any otherreceived probe response and beacon frames. Once the inquiring stationhas processed any responses, or has decided there will be no responses,it may change to another channel and repeat the process. At theconclusion of the inquiry, the station has accumulated information aboutthe wireless LAN cells in its vicinity.

[0022] Joining a wireless LAN cell requires that all of the mobilestation's MAC and physical parameters be synchronized with the desiredwireless LAN cell. To do this, the station must update its timer withthe value of the timer from the wireless LAN cell description, modifiedby adding the time elapsed since the description was acquired. This willsynchronize the timer to the wireless LAN cell. Once this process iscomplete, the mobile station has joined the wireless LAN cell and isready to begin communicating with the stations in the wireless LAN cell.

[0023] Each wireless station and access point in an IEEE 802.11 wirelessLAN implements the MAC layer service, which provides the capability forwireless stations to exchange MAC frames. The MAC frame is a packet thattransmits management, control, or data between wireless stations andaccess points. After a station forms the applicable MAC frame, theframe's bits are passed to the Physical Layer for transmission.

[0024] Before transmitting a frame, the MAC layer must first gain accessto the network. Three interframe space (IFS) intervals defer an IEEE802.11 station's access to the medium and provide various levels ofpriority. Each interval defines the duration between the end of the lastsymbol of the previous frame to the beginning of the first symbol of thenext frame. The Short Interframe Space (SIFS) provides the highestpriority level by allowing some frames to access the medium beforeothers, such as an Acknowledgement (ACK) frame, a Clear-to-Send (CTS)frame, or a subsequent fragment burst of a previous data frame. Theseframes require expedited access to the network to minimize frameretransmissions.

[0025] The Priority Interframe Space (PIFS) is used for high priorityaccess to the medium during the contention-free period. A pointcoordinator in the access point connected to the backbone networkcontrols the priority-based Point Coordination Function (PCF) to dictatewhich stations in a cell can gain access to the medium. The pointcoordinator in the access point sends a contention-free poll frame to astation, granting the station permission to transmit a single frame toany destination. All other stations in the cell can only transmit duringa contention-free period if the point coordinator grants them access tothe medium. The end of the contention-free period is signaled by thecontention-free end frame sent by the point coordinator, which occurswhen time expires or when the point coordinator has no further frames totransmit and no stations to poll.

[0026] The distributed coordination function (DCF) Interframe Space(DIFS) is used for transmitting low priority data frames during thecontention-based period. The DIFS spacing delays the transmission oflower priority frames to occur later than the priority-basedtransmission frames. An Extended Interframe Space (EIFS) goes beyond thetime of a DIFS interval as a waiting period when a bad reception occurs.The EIFS interval provides enough time for the receiving station to sendan acknowledgment (ACK) frame.

[0027] During the contention-based period, the distributed coordinationfunction (DCF) uses the Carrier-Sense Multiple Access With CollisionAvoidance (CSMA/CA) contention-based protocol, which is similar to IEEE802.3 Ethernet. The CSMA/CA protocol minimizes the chance of collisionsbetween stations sharing the medium by waiting a random backoff intervalif the station's sensing mechanism indicates a busy medium. The periodof time immediately following traffic on the medium is when the highestprobability of collisions occurs, especially where there is highutilization. Once the medium is idle, CSMA/CA protocol causes eachstation to delay its transmission by a random backoff time, therebyminimizing the chance it will collide with those from other stations.

[0028] In the IEEE 802.11 Standard, the channel is shared by acentralized access protocol, the Point Coordination Function (PCF),which provides contention-free transfer based on a polling schemecontrolled by the access point (AP) of a basic service set (BSS). Thecentralized access protocol gains control of the channel and maintainscontrol for the entire contention-free period by waiting the shorterPriority Interframe Space (PIFS) interval between transmissions than thestations using the Distributed Coordination Function (DCF) accessprocedure. Following the end of the contention-free period, the DCFaccess procedure begins, with each station contending for access usingthe CSMA/CA method.

[0029] The 802.11 MAC Layer provides both contention and contention-freeaccess to the shared wireless medium. The MAC Layer uses various MACframe types to implement its functions of MAC management, control, anddata transmission. Each station and access point on an 802.11 wirelessLAN implements the MAC Layer service, which enables stations to exchangepackets. The results of sensing the channel to determine whether themedium is busy or idle are sent to the MAC coordination function of thestation. The MAC coordination also carries out a virtual carrier senseprotocol based on reservation information found in the Duration Field ofall frames. This information announces to all other stations the sendingstation's impending use of the medium. The MAC coordination monitors theDuration Field in all MAC frames and places this information in thestation's Network Allocation Vector (NAV) if the value is greater thanthe current NAV value. The NAV operates similarly to a timer, startingwith a value equal to the Duration Field of the last frame transmissionsensed on the medium and counting down to zero. After the NAV reacheszero, the station can transmit if its physical sensing of the channelindicates a clear channel.

[0030] At the beginning of a contention-free period, the access pointsenses the medium; and if it is idle, it sends a beacon packet to allstations. The beacon packet contains the length of the contention-freeinterval. The MAC coordination in each member station places the lengthof the contention-free interval in the station's Network AllocationVector (NAV), which prevents the station from taking control of themedium until the end of the contention-free period. During thecontention-free period, the access point can send a polling message to amember station, enabling it to send a data packet to any other stationin the BSS wireless cell.

[0031] Quality of service (QoS) is a measure of service quality providedto a customer. The primary measures of QoS are message loss, messagedelay, and network availability. Voice and video applications have themost rigorous delay and loss requirements. Interactive data applicationssuch as Web browsing have less restrained delay and loss requirements,but they are sensitive to errors. Non-real-time applications such asfile transfer, email, and data backup operate acceptably across a widerange of loss rates and delay. Some applications require a minimumamount of capacity to operate at all—for example, voice and video. Manynetwork providers guarantee specific QoS and capacity levels through theuse of Service-Level Agreements (SLAs). An SLA is a contract between anenterprise user and a network provider that specifies the capacity to beprovided between points in the network that must be delivered with aspecified QoS. If the network provider fails to meet the terms of theSLA, then the user may be entitled a refund. The SLA is typicallyoffered by network providers for private line, frame relay, ATM, orInternet networks employed by enterprises.

[0032] The transmission of time-sensitive and data application trafficover a packet network imposes requirements on the delay or delay jitter,and the error rates realized; these parameters are referred togenerically as the QoS (Quality of Service) parameters. Prioritizedpacket scheduling, preferential packet dropping, and bandwidthallocation are among the techniques available at the various nodes ofthe network, including access points, that enable packets from differentapplications to be treated differently, helping achieve the differentquality of service objectives. The above-cited, copending U.S. patentapplication, entitled “Tiered Contention Multiple Access (TCMA): AMethod for Priority-Based Shared Channel Access,” describes the TieredContention Multiple Access (TCMA) distributed medium access protocolthat schedules transmission of different types of traffic based on theirQoS service quality specifications.

[0033] For multiple-cell wireless LANs, the limited availability ofchannels implies that the channels must be re-used, much like incellular communication networks. But unlike in cellular networks, thenumber of channels available in wireless LANs is not adequate to ensureboth contiguous coverage (which is essential for roaming) andinterference-free connections at the same time. As a result, cellsassigned the same channel may experience co-channel interference in thearea of overlapping coverage or near a cell's periphery. The problem ofoverlapping cell coverage is acute when wireless LANs are installedwithout any awareness of what other wireless LANs are operating nearby.Consequently, multiple-cell wireless LANs must rely on a medium accesscontrol (MAC) protocol to allocate channel time among stations in orderto avoid co-channel interference between cells, just as it avoidscontention among stations within the same cell.

[0034] Special MAC protocols are provided for wireless LANs becausetransmission is flawed by higher bit error rates, different losses areexperienced on a wireless channel depending on the path on which thesignal travels, and a radio node cannot listen while transmitting.Additive noise, path loss and multipath result in more retransmissionsand necessitate acknowledgements, as successful transmission cannot betaken for granted. The different losses experienced along differentpaths cause different nodes to receive transmissions at differentstrengths, giving rise to the phenomenon of hidden terminals, as isknown in the art. These are terminals that cannot hear or be heard by asource but are capable of causing interference to the destination of atransmission. The message exchange mechanism known in the art asRequest-to-Send/Clear-to-Send (RTS/CTS) alleviates the hidden terminalproblem. RTS/CTS also provides a reservation mechanism that can savebandwidth in wireless LANs. The inability to detect a collision asquickly as it can be detected on cable with carrier-sense multipleaccess with collision detection (CSMA/CD) causes more channel time to bewasted in a collision while waiting for the entire frame to transmitbefore the collision is detected. Hence, carrier sensing is combinedwith the RTS/CTS mechanism to give carrier-sense multiple access withcollision avoidance (CSMA/CA).

[0035] All channel reservations, generated either with an RTS/CTSexchange or for a contention-free period (CFP), are made with the aid ofthe Network Allocation Vector (NAV), which is a timer maintained by allstations. The NAV is set at the value of the duration field broadcastwhen the reservation is announced, either by the RTS or CTS frames, orwith the PCF beacon transmitted by the AP to initiate the CFP. Allstations in a cell defer access until the NAV expires. The NAV thusprovides a virtual carrier-sense mechanism.

[0036] Receiving signals at different strengths, depending on theirorigin, gives rise to capture effects. A known capture effect, the“near-far capture,” results from stronger signals being receivedsuccessfully while other stations transmit at the same time. Near-farcapture leads to inequities, as throughput is greater for nearbystations while distant stations are starved. In infrastructure wirelessLANs, where all communications occur through the AP, the inequity can beremedied by applying power control at the station (i.e., on the uplink).By equalizing the signal strength received at the AP, all transmissionshave equal probabilities of success.

[0037] A special IEEE 802.11 study group is working on enhancements tothe MAC protocols that achieve acceptable QoS for Wireless LANs.Proposals for a QoS enhanced DCF (EDCF) mechanism and a QoS enhanced PCF(EPCF) mechanism are under review.

[0038] The proposed EDCF mechanism employs the Tiered ContentionMultiple Access (TCMA) protocol. The basic access rules of TCMA aresimilar to CSMA with the following differences: transmission deferraland backoff countdown depend on the priority classification of the data.A station still waits for an idle time interval before attemptingtransmission following a busy period, but the length of this interval isno longer equal to DIFS. The length of an idle time interval is equal tothe Arbitration-Time Inter-Frame Space (AIFS), which varies with thepriority of the data. A shorter AIFS is associated with higher prioritydata. As a consequence, higher priority data gets to the channel faster.In addition, countdown of the backoff timer does not commence when abusy period completes unless the channel has been idle for a periodequal to AIFS. This causes backoff countdown of lower priority frames toslow down and even freeze if there are higher-priority frames ready totransmit, a common occurrence in congestion.

[0039] The proposed EPCF maintains multiple traffic queues at thestations for different traffic categories. Higher-priority frames arescheduled for transmission first. Delays are reduced through improvedpolling-list management. Only active stations are kept on the pollinglist. A station with data to transmit must reserve a spot on that list,where it stays as long as it is active and for a limited number ofinactive polling cycles. In the proposed draft standard, the reservationoccurs inside the CFP, using a multi-channel ALOHA channel accessmechanism to forward reservation requests. A priority mask is availableto restrict contention by priority in case of congestion. Several of thefeatures in EPCF are part of the MediaPlex protocol.

[0040] The hybrid coordination function (HCF) has been proposed toprovide a generalization of PCF. It allows for contention-free transfersto occur as needed; not necessarily at pre-determined regular repeattimes as provided by the PCF. The AP can thus send (and possiblyreceive) data to stations in its BSS on a contention-free basis. Thiscontention-free session, referred to as a contention-free burst (CFB),helps an AP transmit its traffic, which is typically heavier ininfrastructure cells (since stations must communicate exclusivelythrough the AP). As in the case of the PCF, the HCF permits access tothe channel by the AP after waiting for an idle period of length equalto PIFS.

[0041] Attention has also been given by the study group to the problemof co-channel overlapping BSSs (OBSSs). Channel re-use in multiple-cellWireless LANs poses a problem for the PCF and HCF, as contention-freesessions (CFSs) are generated without coordination among co-channel APsto help prevent time overlap. Some mechanism is needed in situationswhere cells are within interference range of each other. The existingstandard does not provide adequate coordination for contention-freesessions in such situations. The DCF mechanism does not require specialmeasures, as stations operating under the DCF mechanism deal withinterference from stations in other cells in exactly the same manner asthey deal with interference from stations in their own cell.

[0042] All stations within the cell operate on one duplex TDD channel,with only one station in each cell transmitting data at any given time.In order to preserve power, stations go into a sleeping mode, whichprevents frequent changes of the operating channel. Channel assignmentsshould thus be fixed or static. Static assignments permit slowadaptation to traffic pattern changes over the course of a day. Ideally,these fixed or static assignments must be made optimal through the useof fixed or adaptive non-regular channel assignment methods, which arebased on measurement-derived re-use criteria known in the art. With suchan approach, statistical interference relationships between cells areestablished from measurements of the signal strength between stationsand APs in different cells. Optimization methods use these relationshipsto assign the available channels to cells. Ad hoc channel assignmentmethods, like Dynamic Frequency Selection of HiperLAN2, can be used butwith less promising results, as the re-use distances between co-channelcells are not selected optimally.

[0043] The limited number of channels available in the unlicensed band(three channels for IEEE 802.11b) will lead to a high degree of overlapin the coverage areas of co-channel cells. This overlap is exacerbatedby the ad hoc placement of wireless LANs that results in overlappingBSAs. The channel time (or bandwidth) must thus be allocated amongmultiple co-channel cells in order to avoid interference. To beefficient, the channel should not remain idle if there is data waitingfor transmission. Thus, while channel selection must be fixed or static,bandwidth allocation should be dynamic (possibly changing on aper-transmission basis).

[0044] A distributed dynamic bandwidth allocation mechanism is simply adistributed contention-based MAC protocol, which must enable sharing ofthe channel among APs and DCF stations in co-channel cells, as HCF andDCF co-exist. With APs accessing the channel to initiate contention-freesessions (CFPs or CFBs) before DCF stations, a prioritized distributedMAC protocol is needed. Such a protocol would also handle differentpriority DCF data.

[0045] The priority-based distributed MAC protocol for EDCF, TCMA, canbe used to allocate the channel time among co-channel cells in amultiple-cell wireless LAN. The APs would be treated as a class withpriority above the highest DCF priority class and would be assigned,therefore, a shorter AEFS than the highest-priority EDCF data. Othervariations of CSMA are also appropriate.

[0046] In general, a carrier-sense-based MAC protocol would help avoidinterference between cells as it causes conflicting transmissions—eitherDCF transmissions or CFSs—to occur at statistically (ordeterministically, depending on the protocol) different times inco-channel cells.

[0047] The objective of dynamic bandwidth allocation is to promote fairaccess to the channel for all co-channel cells. That is, the successrate of a cell in accessing its assigned channel, either by its APgenerating CFSs or by (E)DCF transmissions, should be independent of itslocation, assuming comparable traffic loads. Without fair access,transmissions can be delayed excessively in the disadvantaged cell, thusfailing to meet QoS requirements. This goal is not realized with atraditional CSMA-type of protocol, however, when channel re-use isallowed because of a neighborhood capture effect.

[0048] Neighborhood capture arises when Ethernet-type protocols areemployed in multiple-cell wireless LANs that re-use radio frequency (RF)channels. Given the small number of channels available, co-channel cellscannot all transmit simultaneously without causing interference on oneanother. A carrier-sense contention-based MAC protocol can allocatechannel bandwidth among co-channel cells dynamically and in adistributed manner; but if used in the conventional way, it may lead tochannel capture. Mutually non-interfering co-channel neighbors coulddeprive other co-channel neighbors of access. In general, there will beinstability, with the channel retained by a group of cells for long timeintervals. This would have negative impact on quality of service (QoS).

[0049] Neighborhood capture arises in a multiple-cell wireless LAN withfewer channels available than the number of cells. Unlike in cellularcommunications networks, where sufficient channels are available toensure interference-free transmission on an assigned channel, channelselection in WLAN networks must be accompanied by dynamic bandwidthallocation in order to avoid interference between co-channel cells.

[0050] Carrier-sense multiple access (CSMA)-type media access control(MAC) protocols provide dynamic bandwidth allocation in a distributedmanner, obviating the need for a central controller. With suchprotocols, time-overlapped transmissions by stations in non-interferingco-channel cells cooperate to capture the channel for long time periods.The resulting neighborhood capture is deleterious to QoS because of theensuing access delays in other co-channel cells.

[0051] The present invention addresses neighborhood capture andestablishes a method to prevent its occurrence.

SUMMARY OF THE INVENTION

[0052] The neighborhood capture problem described above is mitigated byrequiring that the channel be released by all stations at prespecifiedtimes, ideally regularly spaced. All co-channel cells are thus given anequal opportunity to contend for the channel. Slotted CSMA does noteliminate all inequities; it simply eliminates unfairness due to thesynergy of the cells in a re-use group in capturing the channel, at theexpense of co-channel cells outside that group. Traffic loads must beequally distributed across both cells and re-use groups for all stationsto have the same success rate in seizing the channel. Additionally avariety of schronization schemes can be employed to correct timeoffsets.

[0053] One aspect of the invention is a method to enable overlappingfirst and second wireless LAN cells in a medium to have an equal chanceat establishing a session on the medium. Each cell includes a respectiveplurality of member stations. A first member station in the first celltransmits a timing packet containing a timestamp value, which isreceived at a second member station in the second cell. The timingpacket can be the beacon frame packet or the probe response frame packetof the IEEE 802.11 standard, in which the packet carries a superframetimestamp field. This synchronizes member stations in the first andsecond cells to interrupt transmissions at a global channel releaseinstant corresponding to the timestamp value. The member stations in thefirst and second cells then have the opportunity to contend for accessto the medium following the global channel release instant. Each of themember stations in the first and second cells has a superframe clockthat is synchronized based on the timestamp value. This enablesestablishing a periodic global channel release instant at the memberstations during each of a plurality of periodic superframes based on theclock. The member stations can then periodically interrupt transmissionsat the periodic global channel release instant to contend for themedium. The periodic global channel release instant occurs at intervalsthat are sufficiently close to meet delay and jitter restrictions fortime-critical voice and video applications. The contention for access tothe medium is by a slotted CSMA/CA access method that takes placefollowing the periodic global channel release instant.

[0054] The resulting invention provides a dynamic bandwidth allocationscheme to promote fair access to the channel for all co-channel cells.It enables the success rate of a cell in accessing its assigned channelto be independent of its location, assuming comparable traffic loads.The invention provides a dynamic bandwidth allocation scheme that meetsQoS requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The invention is best described with reference to the detaileddescription and the following figures.

[0056]FIG. 1 shows a three-cell co-channel group.

[0057]FIG. 1A shows the three-cell co-channel group of FIG. 1, in whichstation 3 transmits a beacon packet with a superframe timestamp thatsynchronizes the SF clock in each station, thereby enabling globalchannel release (GCR) by all stations.

[0058]FIG. 1B shows the three-cell co-channel group of FIG. 1A, at alater target beacon transmission time (TBTT) in which Wireless station 5transmits its beacon frame packet 100′, relaying a superframe timestampvalue that is updated with the passage of time since its receipt fromstation 3.

[0059]FIG. 2 depicts capture by re-use group A-C.

[0060]FIG. 3 illustrates a four-cell co-channel group.

[0061]FIG. 4 shows the capture effect mitigated.

[0062]FIG. 5 depicts the global channel release of the present inventionwith one busy period (BP) per frame.

[0063]FIG. 6 illustrates the global channel release of the presentinvention with multiple busy periods (BPs) per superframe.

[0064]FIG. 7 depicts a three-cell co-channel group as depicted in FIG. 1with the addition of the wired distribution network, showing the beaconpacket with the superframe timestamp that synchronizes the SF clock ineach station, thereby enabling global channel release (GCR) by allstations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065] The neighborhood capture effect can be understood by consideringa multiple-cell WLAN where three cells have been assigned the samechannel. The cells assigned to the same channel are referred to as aco-channel group. As illustrated in FIG. 1, the three cells A, B, and Ccomprise nine stations. Cells are depicted as dotted circles. Thestations are labeled numerically and the cells are labeled withalphabetic characters. Stations 1, 2, and 3 make up cell A with station3 serving as the AP for cell A. Stations 4, 5, and 6 make up cell B withstation 4 as cell B's AP. Stations 7, 8, and 9 make up cell C withstation 9 as the AP for cell C. Cells A and C are not withininterference range of each other, so they are called a re-use group.Stations in the pair of cells A-B or B-C are, however, within possibleinterference range of one another. In the A-B cell pair, station 2 ofcell A and station 5 of cell B are both in the possible interferencerange of one another. Correspondingly, in the B-C cell pair, station 6of cell B and station 7 of cell C are both in the possible interferencerange of one another.

[0066] All stations use a CSMA-type of protocol to access the channel,which involves some form of carrier-sensing (either actual or virtual).A station will refrain from transmitting while the channel is busy, andtransmission will be deferred until the backoff timer expires. Backoffcountdown occurs while the channel is sensed idle and an idle timeinterval equal to the AEFS for the priority of the pending transmissionhas elapsed following a busy period.

[0067] Because different cells hear different transmissions, dependingon their location relative to other co-channel cells, their backoffcountdown rates are different. As a consequence, cell B will havedifficulty accessing the channel. In FIG. 2, the periods during whichthe channel is busy as a result of transmissions generated in each cellare shown separately. Busy periods are separated by the ShortInter-Frame Spaces (SIFS) and idle time slots needed for AIFS andbackoff delay. (AIFS equals a SIFS plus a variable number of timeslots.) A station in cell A may transmit at the same time as stations incell C. Stations in cell A must refrain from transmitting only whenstations in cell B are transmitting. Stations in cell B are preemptedfrom accessing the channel by transmissions in either of its interferingneighbors, cells A or C.

[0068] Because transmissions have variable lengths, it is very likelyunder loaded traffic conditions for a station in cell A to start atransmission before a transmission in cell C expires, and vice versa. Asa result, cells A and C will capture the channel, not allowing a chancefor stations in cell B to transmit. In general, one would expect thatperiphery cells, or cells at the top or bottom floors of amultiple-story building equipped with a multiple-cell WLAN, to be likelyto capture the channel, at the expense of cells in the same co-channelgroup located in the interior. In this instance, a cell is disadvantagednot only because its competition for the channel—namely, the re-usegroup comprising cells A and C—has a greater combined offered load, butalso because selected station members of a re-use group may transmitsimultaneously, thus prolonging their retention of the channel. As canbe seen from FIG. 2, the only traffic from cell B to be transmitted washigh-priority. All other traffic from cell B was blocked by traffic fromcells A and C even though the traffic from cells A and C was medium-andeven low-priority.

[0069] Even when all cells have the same degree of competition fromneighbors in the same co-channel group, there is still a problem.Consider the situation in FIG. 3 illustrating four cells, each cellhaving two competing co-channel neighbors. Once again the cells arelabeled with alphabetic characters, the stations are labeled numericallyand the cells are depicted by dotted circles. Two re-use groups exist inthis co-channel group of cells: one comprising cells A and C, andanother comprising cells B and D. As before, under loaded trafficconditions, a station in cell A may start a transmission beforecompletion of a transmission in cell C, thus failing to release thechannel for access by cells B and D. The same is true for stations incells B and D. If a station in either cell seizes the channel, it willnot be released until there is no pending traffic in the other cell.Assuming the offered loads in the two groups of non-interfering cellsare the same, they have equal probability of capturing the channel;hence, there is no a priori unfairness in this instance. Once thechannel is accessed by one re-use group, however, it will be capturedand the other group deprived access. In general, instability in channelaccess would result, with long channel retention periods by each re-usegroup.

[0070] The neighborhood capture effect will be worse if traffic loadsare balanced across cells, as the synergy of cells in the same re-usegroup is maximum in that case. Equal traffic loading across cells isdesirable for fair access in a multiple-cell WLAN, as the probability ofaccessing the channel successfully within a cell decreases withincreasing load. In order to avoid unfairness, it is desirable to sizecells (through AP power adjustment) so that the traffic loads in allcells are equal. Load balancing magnifies the negative impact ofneighborhood capture, however, as the channel will be released only ifthere is no pending traffic in another cell of the same re-use group. Are-use group thus achieves the maximum retention probability if itscombined load is equally split among its members.

[0071] Neighborhood capture has a negative impact on QoS delivery.Transmissions in cells outside the re-use group capturing the channelwill be delayed excessively as transmissions will find the channel busyfor long time intervals. In consequence, CFPs could not be initiated asscheduled and periodic and time-critical data will be delayed. Theprioritization apparatus put in place for EDCF will also be renderedineffective.

[0072] Neighborhood capture can be reduced or eliminated by requiringall stations to release the channel at prespecified times. All competingco-channel cells would thus have an equal chance to seize the channel.Global channel release (GCR) should occur at regularly spaced timeintervals that are sufficiently close to meet delay and jitterrestrictions for time-critical applications such as voice or video. Thisimplies slotting of the channel into superframes and synchronization ofall stations. The resulting protocol would be a Slotted CSMA/CA. FIG. 4shows how neighborhood capture is mitigated in the three-cell co-channelscenario of FIG. 1 as a result of the global channel release requirementof the present invention.

[0073] As shown in FIG. 1A, the wireless station 3 transmits a timingpacket, such as the beacon frame packet 100 or probe response framepacket of the IEEE 802.11 standard, carrying a superframe timestampfield. Each station receiving the timing packet 100 updates its SF clockif the received timestamp is later than the current value of the clock.The initial setting of the clock when a station powers on is 0. Allstations in an IBSS cell prepare to transmit a beacon frame packet at atarget beacon transmission time (TBTT). Each station prepares its beaconpacket to contain the superframe timestamp value. Each station selects arandom delay when it is to transmit its superframe timestamp value. Inthis manner, the superframe timestamp value is propagated to overlappedstations, such as wireless station 5 in FIG. 1A. At a later targetbeacon transmission time (TBTT) of FIG. 1B, wireless station 5 willrelay a superframe timestamp value that is updated with the passage oftime since its receipt, when it transmits its beacon frame packet 100′.In this manner, wireless stations, such as station 9, which may be outof range of wireless station 3, will receive an updated superframetimestamp value. Other timing packets that can propagate the updatedsuperframe timestamp value include the probe response frame packet andthe contention-free time response (CFTR) packet described in thecopending patent application entitled “HYBRID COORDINATION FUNCTION(HCF) ACCESS THROUGH TIERED CONTENTION AND OVERLAPPED WIRELESS CELLMITIGATION,” which is incorporated by reference.

[0074] The MAC protocol allocates channel time among the following:control and data frames and contention-free sessions (both CFPs andCFBs). CFSs may include all frame exchange sequences generated withoutcontention following a successful channel contention-based accessattempt, where contention is avoided through the use of SIFS spacing. ACFS may involve one or more stations and may be initiated by anystation. For simplicity, the generic term busy period (BP) is used todesignate any of the above.

[0075] Different MAC protocols may be used to access the channel for thedifferent BP types. All of the MAC protocols, however, are distributedand based on carrier-sensing. BPs are all assigned an AEFS; EDCFstations are assigned AEFS values according to their priorityclassification; CFPs and CFBs are assigned the shortest possible AIFSvalue.

[0076] Capture is mitigated by requiring that the channel be free of allactivity at prespecified times—termination of the busy period (TBPend).The channel time is slotted at equal time intervals, thus creatingsuperframes (SF) of duration SFDuration. BPs may complete before thenext frame boundary. As a result, the BPs that follow must beforeshortened in order to ensure termination of the BP at the designatedslotted time. This length adjustment will account also for idle timespent for AIFS and backoff delay. FIG. 5 illustrates how equal-size BPsof length BPLength are foreshortened in order to meet the global channelrelease requirement. Stations that attempt access unsuccessfully using ashort reservation packet (RTS/CTS) may be able to use the channel withinthe same superframe. After first engaging in backoff, stations maytransmit concurrently with successful transmissions, provided that theydo not interfere. Their BPs may have to be foreshortened in order torelease the channel at the next TBPend, however.

[0077] GCR does not eliminate all inequities. By forcing stations to endtheir BPs at the same time, equal access is offered to all stations inall cells, as there is no synergy of member cells of the same re-usegroup in retaining the channel. If traffic loads are equally distributedacross cells and re-use groups, all stations have a fair/equal chance atthe channel. But if the combined offered load is greater in one re-usegroup, as is possible for instance with group A-C which has morestations, the success rate of cell B would be less. GCR improves thesuccess rate of cell B, however, relative to what the success rate wouldhave been otherwise. To achieve greater fairness, traffic loads in allre-use groups must be comparable—hence the need to balance loads notonly across cells, but also across re-use groups.

[0078] It is not necessary for GCR to occur after each data frame orCFS; it may happen less often. FIG. 6 illustrates BPs of maximum-lengthBPLength shorter than the superframe duration. In general, there may bemultiple BPs per superframe.

[0079] In order to avoid their BPs straddling the superframe boundary,all stations in the multiple-cell WLAN must be synchronized.Synchronization may be achieved in several ways. For instance, within acell, stations may synchronize with the AP, as is done in the currentIEEE 802.11 standard. Neighboring cells may be synchronized via framessent by stations in the overlapping coverage area of two cells. However,time offsets may arise between different cells as distant cells [cellsthat cannot hear each other] power on and synchronize locally,independently of one another. This would happen early in the morningwhen few stations are on. As more stations power on and synchronize withtheir neighbors in the course of the day, asynchrony may arise. Clockadjustment is necessary in order to eliminate time offsets.

[0080] Time offsets between cells may be corrected in a way similar tonode synchronization in an independent BSS. As shown in FIG. 7, thestations transmit a special frame, such as the beacon packet 100 orprobe response frame of IEEE 802.11, carrying a superframe timestampfield. Each station updates its SF clock if the received timestamp islater. The initial setting of the clock when a station powers on is 0.

[0081] Other mechanisms are also possible for synchronization. Forinstance, synchronization between cells may be pursued through the wireddistribution system in infrastructure WLANs.

[0082]FIG. 7 depicts a three-cell co-channel group similar to thatdepicted in FIG. 1 with the addition of the wired distribution network110, which is in communication with each cell via the cell's AP.Synchronization between cells can be maintained by transmitting thesuperframe timestamp over the wired distribution system 110.Additionally, stations in each cell can communicate with each other andwith stations in other cells via the AP, which is in communication withthe wired distribution network 110. For example, if station 1 in cell Ahas data/communication traffic for station 7 in cell C, then whenstation 1 is polled by AP 3 of cell A, station 1 indicates that it hasdata for station 7 of cell C and the priority of the data. The AP 3 canthen poll station 1 to send the data to the AP 3. If AP 3 is withinwireless communication range of AP 9 in cell C, it can attempt to gainchannel access to the wireless medium to communicate that data to AP 9.If channel access is granted, then AP 3 of cell A forwards station 1'sdata frames over the wireless medium to AP 9. Alternately, AP 3 canaccess the wired distribution network to forward the frames to station 9of cell C for final distribution to station 7 of cell C. The AP can usethe IEEE 802.3 protocol to access the wired distribution network 110.

[0083] The phenomenon of neighborhood capture, which arises in amultiple-cell wireless LAN with fewer channels available than the numberof cells, has been described herein. Unlike in cellular communicationsnetworks where sufficient channels are available to ensureinterference-free transmission on an assigned channel, channel selectionin WLAN networks must be accompanied by dynamic bandwidth allocation inorder to avoid interference between co-channel cells.

[0084] CSMA-type MAC protocols provide dynamic bandwidth allocation in adistributed manner, obviating the need for a central controller. Withsuch protocols, time-overlapped transmissions by stations innon-interfering co-channel cells cooperate to capture the channel forlong time periods. The result is deleterious to QoS because of theensuing access delays in other co-channel cells.

[0085] The problem of neighborhood channel capture can be mitigated byrequiring that the channel be released by all stations at prespecifiedtimes, ideally regularly spaced. All co-channel cells are thus given anequal opportunity to contend for the channel. Slotted CSMA does noteliminate all inequities, but rather simply eliminates unfairness due tothe synergy of the cells in a re-use group in capturing the channel atthe expense of co-channel cells outside that group. Traffic loads mustbe equally distributed (balanced) across both cells and re-use groupsfor all stations to have the same success rate in seizing the channel.

[0086] It should be clear from the foregoing that the objectives of theinvention have been met. While particular embodiments of the presentinvention have been described and illustrated, it should be noted thatthe invention is not limited thereto since modifications may be made bypersons skilled in the art. The present application contemplates any andall modifications within the spirit and scope of the underlyinginvention disclosed and claimed herein.

What is claimed is:
 1. A method to enable overlapping first and secondwireless LAN cells in a medium to have an equal chance at establishing asession on the medium, each cell including a respective plurality ofmember stations, comprising: transmitting by a first member station inthe first cell, a timing packet containing a timestamp value; receivingthe timing packet at a second member station in the second cell;interrupting transmissions by member stations in the first and secondcells at a global channel release instant corresponding to saidtimestamp value; and permitting member stations in the first and secondcells to contend for access to said medium following said global channelrelease instant.
 2. The method of claim 1, which further comprises:synchronizing a superframe clock in member stations in the first andsecond cells based on said timestamp value; establishing a periodicglobal channel release instant at member stations in the first andsecond cells during each of a plurality of periodic superframes based onsaid clock; and periodically interrupting transmissions at memberstations in the first and second cells at said periodic global channelrelease instant.
 3. The method of claim 2, which further comprises:periodically permitting member stations in the first and second cells tocontend for access to said medium following said periodic global channelrelease instant.
 4. The method of claim 3, which further comprises: saidperiodic global channel release instant occurring at intervals that aresufficiently close to meet delay and jitter restrictions fortime-critical voice applications.
 5. The method of claim 3, whichfurther comprises: said periodic global channel release instantoccurring at intervals that are sufficiently close to meet delay andjitter restrictions for time-critical video applications.
 6. The methodof claim 3, which further comprises: said member stations in the firstand second cells contending for access to said medium using a slottedCSMA/CA access method following said periodic global channel releaseinstant.
 7. The method of claim 6, which further comprises: conducting adistributed coordination function (DCF) session by one of said memberstations in the first and second cells that has successfully accessedthe medium following said periodic global channel release instant. 8.The method of claim 6, which further comprises: conducting a TieredContention Multiple Access (TCMA) session by one of said member stationsin the first and second cells that has successfully accessed the mediumfollowing said periodic global channel release instant.
 9. The method ofclaim 6, which further comprises: conducting a point coordinationfunction (PCF) session by one of said member stations in the first andsecond cells that has successfully accessed the medium following saidperiodic global channel release instant.
 10. The method of claim 6,which further comprises: requiring that the medium be free of allactivity at the termination of a busy period; detecting completion of abusy period before a next periodic global channel release instant; andforeshortening a subsequent busy period in order to ensure terminationof the subsequent busy period at a next periodic global channel releaseinstant.
 11. A method of distributed dynamic bandwidth allocation formitigating the effects of neighborhood capture in an infrastructureWLAN, comprising the steps of: time-slotting of a communications channelinto superframes of superframe duration (SFDuration) by a MAC protocol,wherein said superframes comprise control and data frames and busyperiods (BPs), wherein said BPS comprise contention-free periods (CFPs)and contention-free bursts (CFBs); assigning each BP an Arbitration-TimeInter-Frame Space (AEFS); further assigning each enhanced distributedcoordination function (EDCF) station AEFS values according to saidstation's priority classification; further assigning CFPs and CFBs theshortest possible AEFS values; further assigning available channels tocells; polling stations within a cell by an access point (AP) todetermine if said stations within said cell have communications trafficto transmit; attempting to gain access to said communications channel bymeans of a contention-based scheme using a short reservation packet bysaid AP if said AP determined that said stations within said cell hadcommunications traffic to transmit; performing frame exchanges ifchannel access is gained; gaining access to said communications channelby one of waiting for a backoff period and re-attempting to gain accessto said communication channel in the event that a transmission byanother cell is less than a full time slot if channel access was notgained and waiting to attempt to gain access until a next time slot;performing a global channel release (GCR) of all channels atpre-specified intervals; and continuing to perform the above steps. 12.The method according to claim 11, wherein said contention-based schemeis based on carrier-sensing.
 13. The method according to claim 11,wherein said BPs are foreshortened if necessary to meet GCRrequirements.
 14. The method according to claim 11, wherein said GCR mayoccur less frequently than after each BP if BPs are shorter than saidSFDuration.
 15. The method according to claim 11, wherein saiddistributed dynamic bandwidth allocation scheme gives all co-channels anequal opportunity to contend for said communication channel.
 16. Themethod according to claim 11, wherein said GCR is performed at regularlyspaced intervals.
 17. The method according to claim 11, whereincommunication traffic loads are balanced across cells and re-use groups.18. The method according to claim 11, wherein cells in a multiple cellWLAN are synchronized to avoid BPs straddling a boundary of saidsuperframe.
 19. The method according to claim 18, wherein stationswithin a cell are synchronized with said cell's access point (AP). 20.The method according to claim 18, wherein neighboring cells synchronizewith frames sent by stations in an overlapping coverage area of saidneighboring cells.
 21. The method according to claim 18, wherein timeoffsets are corrected by stations transmitting a special frame carryinga timestamp field.
 22. The method according to claim 21, wherein eachstation updates its clock upon receipt of said special frame having saidtimestamp field.
 23. The method according to claim 19, whereinsynchronization between cells is accomplished via a wired distributionsystem in communication with said APs.
 24. A multiple-cell wirelesslocal area network including overlapping first and second wireless LANcells in a medium, each cell including a respective plurality of memberstations, comprising: a first member station in the first cell,transmitting a timing packet containing a timestamp value; a secondmember station in the second cell receiving the timing packet; saidmember stations in the first and second cells interrupting transmissionsat a global channel release instant corresponding to said timestampvalue; and said member stations in the first and second cells contendingfor access to said medium following said global channel release instant.25. The network of claim 24, which further comprises: said memberstations in the first and second cells synchronizing a superframe clockbased on said timestamp value; said member stations in the first andsecond cells establishing a periodic global channel release instantduring each of a plurality of periodic superframes based on said clock;and said member stations in the first and second cells periodicallyinterrupting transmissions at said periodic global channel releaseinstant.
 26. The network of claim 25, which further comprises: saidmember stations in the first and second cells periodically contendingfor access to said medium following said periodic global channel releaseinstant.
 27. The network of claim 26, which further comprises: saidperiodic global channel release instant occurring at intervals that aresufficiently close to meet delay and jitter restrictions fortime-critical voice applications.
 28. The network of claim 26, whichfurther comprises: said periodic global channel release instantoccurring at intervals that are sufficiently close to meet delay andjitter restrictions for time-critical video applications.
 29. Thenetwork of claim 26, which further comprises: said member stations inthe first and second cells contending for access to said medium using aslotted CSMA/CA access method following said periodic global channelrelease instant.
 30. The network of claim 29, which further comprises:said member stations in the first and second cells detecting completionof a busy period before a next periodic global channel release instant;and said member stations in the first and second cells foreshortening asubsequent busy period in order to ensure termination of the subsequentbusy period at a next periodic global channel release instant.
 31. Thenetwork of claim 29, which further comprises: one of said memberstations in the first and second cells conducting a distributedcoordination function (DCF) session, said one of said member stationshaving successfully accessed the medium following said periodic globalchannel release instant.
 32. The network of claim 29, which furthercomprises: one of said member stations in the first and second cellsconducting a Tiered Contention Multiple Access (TCMA) session, said oneof said member stations having successfully accessed the mediumfollowing said periodic global channel release instant.
 33. The networkof claim 29, which further comprises: one of said member stations in thefirst and second cells conducting a point coordination function (PCF)session, said one of said member stations having successfully accessedthe medium following said periodic global channel release instant.
 34. Amultiple-cell wireless local area network (WLAN) using distributeddynamic bandwidth allocation for mitigating the effects of neighborhoodcapture, comprising: a wired distribution network; and a plurality ofcells, each cell having a plurality of stations, wherein one of saidstations in each cell is an access point (AP), and further whereincommunications between said plurality of stations is via said AP andwhere said plurality of stations are within one of a same cell and adifferent cell, and further wherein communications between one of saidstations and said wired distribution network is via said AP, and furtherwherein stations in some different cells overlap; wherein said wireddistribution network allocates time-slots of a communications channelinto superframes of superframe duration (SFDuration) using a MACprotocol, wherein said superframes comprise control frames and dataframes and busy periods (BPs), wherein said BPs comprise contention-freeperiods (CFPs) and contention-free bursts (CFBs); further wherein saidwired distribution network assigns each BP an Arbitration-TimeInter-Frame Space (AEFS); further wherein said wired distributionnetwork assigns each enhanced distributed coordination function (EDCF)station AIFS values according to said station's priority classification;further wherein said wired distribution network assigns CFPs and CFBsthe shortest possible AIFS values; further wherein stations within eachcell are polled by their respective AP to determine if said stationswithin said cell have communications traffic to transmit; furtherwherein each said AP attempts to gain access to said communicationschannel by means of a contention-based scheme using a short reservationpacket if each said AP determines that said stations within said cellhave communications traffic to transmit; further wherein each said APthat gained access to said communications channel performs frameexchanges with said wired distribution network; further wherein said APthat had communications traffic to transmit and that did not gain accessto said communications channel re-attempts to gain access to saidcommunications channel by one of waiting for a backoff period andattempting to gain access to said communication channel in the eventthat a transmission by another cell is less than a full time slot andwaiting to attempt to gain access until a next time slot; furtherwherein each said AP and said wired distribution network perform aglobal channel release (GCR) of all channels at pre-specified intervals.35. The WLAN according to claim 34, wherein said contention-based schemeis based on carrier-sensing.
 36. The WLAN according to claim 34, whereinsaid BPs are foreshortened if necessary to meet GCR requirements. 37.The WLAN according to claim 34, wherein said GCR may occur lessfrequently than after each BP if BPs are shorter than said SFDuration.38. The WLAN according to claim 34, wherein said distributed dynamicbandwidth allocation scheme gives all co-channels an equal opportunityto contend for said communication channel.
 39. The WLAN according toclaim 34, wherein said GCR is performed at regularly spaced intervals.40. The WLAN according to claim 34, wherein communication traffic loadsare balanced across cells and re-use groups.
 41. The WLAN according toclaim 34, wherein cells in said multiple-cell WLAN are synchronized toavoid BPs straddling a boundary of said superframe.
 42. The methodaccording to claim 41, wherein stations within a cell are synchronizedwith said cell's access point (AP).
 43. The WLAN according to claim 41,wherein neighboring cells synchronize with frames sent by stations in anoverlapping coverage area of said neighboring cells.
 44. The WLANaccording to claim 41, wherein time offsets are corrected by stationstransmitting a special frame carrying a timestamp field.
 45. The WLANaccording to claim 44, wherein each station updates its clock uponreceipt of said special frame having said timestamp field.
 46. The WLANaccording to claim 45, wherein synchronization between cells isaccomplished via a wired distribution system in communication with saidAPs.